1. Field of the Invention
The present invention relates to a lithium polymer battery, and more particularly, to a lithium polymer secondary battery using a hybrid polymer electrolyte and a manufacturing method thereof.
2. Description of the Related Art
Recently, according to the development of portable electronic apparatus such as cellular phones, notebook-type computers, camcorders and the like, research into secondary batteries capable of charging has been vigorously carried out.
Secondary batteries are classified into a variety of batteries, including nickel-cadmium (Nixe2x80x94Cd) batteries, lead storage batteries, nickel metal hydride (Nixe2x80x94MH) batteries, lithium secondary batteries, air-zinc storage batteries, and the like.
Among the above-mentioned batteries, lithium secondary batteries have a service life 3 times longer than Nixe2x80x94Cd batteries or Nixe2x80x94MH batteries which operate with 3.6 V and are widely used as the power source for electronic devices, and have excellent energy density per unit weight.
Lithium secondary batteries are fabricated by providing a cathode, an anode, and an organic electrolyte solution and a separator for supplying a passage for lithium ions between the cathode and the anode. Also, in the lithium secondary battery, electrical energy is generated by oxidation/reduction which takes place when the lithium ions are intercalated or deintercalated into/from the cathode or anode.
Lithium secondary batteries are classified into lithium ion batteries using a liquid electrolyte and lithium polymer batteries using a polymer electrolyte. Here, the polymer electrolyte for use in lithium secondary batteries includes pure solid polymer electrolyte, a gel polymer electrolyte and a hybrid polymer electrolyte.
The hybrid polymer electrolyte is prepared by filling an electrolyte into a porous polymer matrix having a submicron dimension or smaller.
The hybrid polymer electrolyte is used for increasing the workability by forming pores for injection of an electrolyte and imparting flexibility to electrode plates, and there has been proposed a method in which a plasticizer such as dibutyl phthalate (DBP) is added to an electrode active material composition and the plasticizer is extracted and removed with an organic solvent, such as ether, before assembling a battery.
In this connection, a method of manufacturing a lithium polymer battery employing a hybrid polymer electrolyte, which was proposed in U.S. Pat. Nos. 5,460,904, 5,478,668 and 5,705,297 by Bellcore Inc., of U.S.A., will now be briefly described.
First, a unit cathode plate and a unit anode plate are fabricated.
To this end, an acetone solvent, an electrode active material such as a cathode active material or an anode active material, a binder, a conductive agent and a plasticizer are mixed to form slurry. The slurry is made into a thin-film on a polyethylene (PET) base film with by a doctor-blade. Then, the PET base film is removed to produce an electrode sheet of the cathode or anode active material.
The cathode sheet is adhered to a cathode current collector and the anode sheet is adhered to an anode current collector, by applying heat and pressure through a laminating process, respectively. Here, the cathode or anode current collector is made of expanded metal having a plurality of openings formed by expansion due to application of external tension. The openings allow the plasticizer to be easily extracted in a unit battery cell having a bi-cell structure to be described later.
Then, the resultant structure is cut into a predetermined size to form the unit cathode plate and the unit anode plate.
Next, a unit cathode plate, a separator, a unit anode plate, a separator and a unit cathode plate are laminated sequentially and a laminating process is performed by applying heat and pressure to form a unit battery cell having a bi-cell structure.
Then, the unit battery cell having a bi-cell structure is immersed in an ether solution to extract the plasticizer. Here, the plasticizer extracting process is widely used in manufacturing PE or PP separators for use in lithium polymer secondary batteries or in fabricating woolen products.
Next, nine sheets of unit battery cells having a bi-cell structure are laminated to form 9 bi-cells and parallel-connected.
Then, a space produced by extracting the plasticizer is impregnated with an electrolyte solution and encased.
However, in the process of manufacturing the lithium polymer batteries, it is quite difficult to treat plasticizer-containing separators, impurities are easily adsorbed into the separators having viscosity due to the plasticizer, and the processing time is long due to extracting and drying the plasticizer in a bi-cell state, thereby noticeably lowering the manufacturing performance. Also, while performing extraction after laminating the bi-cell structure, bubbling may occur.
Also, in the case of manufacturing lithium polymer batteries using expanded metal as a current collector such as a cathode current collector or an anode current collector, since the directions of elongation of expanded metal and tension applied thereto are the same, permanent deformation may be caused due to elongation of the current collector in the continuous fabrication process of electrode plates such as cathodes or anodes. Thus, the process of manufacturing lithium polymer batteries using expanded metal as a current collector, is not compatible with mass production based on the continuous fabrication process.
Further, in the case of manufacturing lithium polymer batteries using a current collector of expanded metal, the batteries become thick. Also, after the current collector is elongated, the difference in the thickness is 10 xcexcm or greater in regions of a 35 xcexcm thick current collector. Thus, it is difficult to achieve thickness control in the course of adhering an electrode sheet to the current collector.
Instead of using a current collector of expanded metal, a current collector of punched metal having small pores or a foil may be used as a current collector in manufacturing a battery cell having a bi-cell structure. However, in this case, while the electrical conductivity is improved, the plasticizer cannot be easily extracted from the battery cell.
To solve the above problems, it is an object of the present invention to provide a lithium polymer secondary battery using a hybrid polymer electrolyte, which can employ a current collector of expanded metal or punched metal having small pores as well as a foil, and a manufacturing method thereof.
Accordingly, to achieve the above object, there is provided a method for manufacturing a lithium polymer secondary battery including the steps of fabricating a unit cathode plate, a unit anode plate and a separator each having a plasticizer, extracting each plasticizer from the unit cathode plate, the unit anode plate and the separator and drying the same, stacking the cathode plate and the unit anode plate and interposing the separator therebetween to form a unit battery cell, and impregnating the unit battery cell with an electrolytic solution.
Here, the unit cathode plate may be fabricated by forming a cathode active material layer on at least one surface of a cathode current collector and cutting the same into a predetermined size, and the unit anode plate may be fabricated by forming anode active material layer on at least one surface of an anode current collector and cutting the same into a predetermined size.
Also, the cathode active material layer may formed by laminating a cathode sheet made of a cathode active material slurry on the cathode current collector, or by directly coating a cathode active material slurry on the cathode current collector.
Also, the anode active material layer may be formed by laminating an anode sheet made of an anode active material slurry on the anode current collector, or by directly coating an anode active material slurry on the anode current collector.
Further, the anode current collector is preferably shaped of expanded metal, punched metal or a foil. Also, the cathode current is shaped of expanded metal, punched metal or a foil.
The battery cell may be constructed by a plurality of parallel-connected unit battery cells having a bi-cell structure.
According to another aspect of the present invention, there is provided a lithium polymer secondary battery having a unit cathode plate, a unit anode plate, a separator for insulating the unit cathode plate and the unit anode plate from each other, and a hybrid polymer electrolyte for supplying a movement passage of lithium ions, the lithium polymer secondary battery made by the method of the first aspect of the present invention, wherein the unit cathode plate has a cathode active material layer formed on at least one surface of a foil-shaped cathode current collector, and the unit anode plate has an anode active material layer formed on at least one surface of a foil-shaped anode current collector.